Streptococcus pneumoniae (S. pn) is one of the most important causes of death around the world, and is an important pathogenic bacterium for invasive and noninvasive infection such as pneumonia, meningitis, and otitis media. Streptococcus pneumoniae resides in healthy human, typically colonizing pharynx nasalis, and about 40%-70% of humans carry the bacterium. When the body's immune function weakens, the pathogenic bacterium may spread to the lung and cause pneumonia. Besides the carriage, one can also acquire pneumococcal infection by contacting other persons or patients carrying the bacterium.
Vaccination is an effective and specific preventive means, and has an excellent health-economic value. Currently, the widely applied vaccines for preventing pneumococcal diseases mainly include two classes, i.e., polysaccharide vaccines (23-valent polysaccharide vaccine, suitable for people above 2 years old) and polysaccharide-protein conjugate vaccines (7-valent, 10-valent or 13-valent, suitable for infants under 2 years old).
Pneumococcal polysaccharide vaccine refers to “23-valent pneumococcal polysaccharide vaccine”, which can cover 23 serotypes that generally cause about 90% of pneumococcal infection. Most of healthy adults can generate protective antibodies against all or most of Streptococcus pneumonia strains 2-3 weeks after vaccination. Since the 23-valent pneumococcal vaccine can effectively prevent pneumonia, it has been used in more than 30 countries and regions, including USA and Canada, for more than 14 years. The vaccine has been shown to have a 92% protective efficacy and a good safety record. The protection can last for at least five years. It is suggested that children at the age of 2 or older with poor immunity or with recurrent pneumonia as well as the high risk population (such as children with asplenia) should be immunized with the vaccine.
The capsular polysaccharide antigens comprised in 23-valent pneumococcal polysaccharide vaccine are T cell independent antigens, which can stimulate mature B lymphocytes, but not T lymphocytes. The immune response mediated by such antigens only lasts for a short time, and cannot produce immunological memory. Since the immune function in infants under 2 years old is not well developed yet and has a poor response to T cell independent antigens, polysaccharide vaccines cannot induce an effective protective immune response in infants. Therefore, the 23-valent pneumococcal polysaccharide vaccine cannot be used in this high risk population.
7-valent pneumococcal polysaccharide-protein conjugate vaccine can prevent the diseases caused by 7 serotypes of Streptococcus pneumoniae covered by the vaccine. There are researches showing that the diseases caused by said 7 serotypes comprised therein account for about 80% of all pneumococcal diseases. In 2006-2007, in four typical children hospitals in China (i.e., Beijing Children Hospital, Children's Hospital of Fudan University, the Children's Hospital in Guangzhou and the Children's Hospital in Shenzhen), 279 strains of Streptococcus pneumonia were isolated in clinic from children with pneumonia under 5 years old, and it is found that the main serotypes of these clinical isolates are covered by PCV7, and said 7 serotypes account for about 81% of all the pathogenic pneumococcal strains. The results of the study demonstrated that the 7-valent pneumococcal conjugate vaccine has a relatively good serotype coverage in China. However, the 7-valent pneumococcal conjugate vaccine cannot prevent the infection caused by serotypes of Streptococcus pneumoniae other than said 7 serotypes, and there are researches showing that the infection by other serotypes of Streptococcus pneumonia also exists commonly and is rising.
Multiple membrane proteins on the cell surface of Streptococcus pneumonia are important virulence factors, and can also be important antigens. Since many outer membrane proteins have conservative constitutive structures among different serotypes of Streptococcus pneumonia, they can therefore induce cross immune protection. Pneumococcal surface protein A (PspA) is widely present in more than 90 subtypes of Streptococcus pneumonia. The α-helical terminus of the PspA protein is an important antigen determinant region and is highly conservative in different serotypes. Depending on the terminal amino acid sequence, the PspA proteins can be divided into three families, and further divided into six clades.
There are a lot of researches reporting that immunization with the PspA protein is protective against different serotypes of S.pn, and inclusion of the PspA protein as a vaccine component can overcome the shortcomings associated with the polysaccharide-protein conjugate vaccines, which only generate protections against a limited number of capsular serotypes and antibodies raised against the protein carrier lack specific protection against pathogenic bacteria. Epidemiological investigation shows that the PspA proteins from almost all pneumococcal strains isolated in clinic belong to family I and II, and the strains with the PspA proteins belonging to family III are rarely isolated.
Pneumolysin (Ply) mainly plays a role in hemolysis and complement activation Immunization of mice with the recombinant Ply, followed by challenging of Streptococcus pneumonia in nasal cavity and peritoneal cavity, demonstrate that Ply can prolong the life span of mice by 89% and 93%. The amino acid sequences of Ply are highly conservative and the protein is highly immunogenic. Therefore, Ply has been studied in different labs as a vaccine candidate which may potentially cover all the serotypes.
Although PspA and Ply have been shown to be good vaccine candidates, it is not clear yet how the proteins will interact with each other when both are present in the same formulation. In addition, PspA proteins are divided into several families and clads, and it is generally accepted that two or three PspA proteins from family I and II should be included in the vaccine formulation. In order to prepare a vaccine with a wide coverage and efficient protection against pneumococcal infections, it is of great significance to screen out protein immunogenic compositions involving multiple PspA proteins as well as the Ply protein.
Therefore, the invention provides an immunogenic composition, which will cover more than 95% of the serotypes isolated in clinic, when the immunogenic composition is used as antigen component.